The production of antibiotics by plankton algae and its effect upon bacterial activities 2X3 



the bottle : volume of the water — is high, give rise to a faster bacterial decomposiiion 

 than bigger bottles, in which the ratio is lower. The addition of glass beads in the 

 same way increases the bacterial activity. According to ZoBii.i. ( 1946) the influence 

 of the surfaces may be explained (I) through accumulation of the organic matter due 

 to surface activities, (2) by assuming that exoenzymes necessary for attacking the 

 organic matter can only be maintained in sufficient concentration if solid surfaces 

 are present. 



In natural sea water, solid surfaces are of course found. The phytoplankton. 

 zooplankton, and particulate dead material all have solid surfaces. In oceanic sur- 

 face water these surfaces are, however, scarcely more than about 10 mm- per litre. 

 In water from deeper layers they are much smaller. The inner surface of a one-litre 

 bottle is about 10* as large. 



The surfaces of plankton algae seem normally to be rather unsuitable for bacterial 

 growth. If plankton algae are growing well, very few bacteria — if any— can be 

 observed to be attached to their surfaces. If the surfaces of the algae are densely 

 covered with bacteria, this indicates that they are not in a healthy state. The whole 

 procedure of growing algae in a pilot plant, where it is impossible to keep the culture 

 sterile, is based upon the fact that the culture itself is able to keep the bacterial 

 activities at a minimum. This has been known for a long time. Thus Waksman et al. 

 (1937) state that " The result . . . confirmed the previous observation that, in the 

 presence of a living culture of Nitzschia (a plankton algae), the bacterial activities 

 were very limited." 



The statement above does not contradict the fact, often observed, that bacteria 

 mostly are more abundant where a high phytoplankton population is found (e.g. 

 Gran, 1933). The heterotrophic micro-organisms are first of all numerous at the end 

 of a phytoplankton bloom. Beside bacteria, heterotrophic flagellates are also found. 

 Due to the bloom of autotrophic organisms, relatively large quantities of easily 

 accessible organic matter first of all originating from dying and half-digested cells, 

 are contributed to the water. Solid surfaces apparently are not necessary for the 

 bacteria attacking this easily accessible organic matter (no exoenzymes necessary?). 

 They can thus also be in a real planktonic state. 



It can be stated that the dissolved organic matter in sea water consists o\' the 

 following components: 



(1) matter very easily accessible for bacteria, (2) matter only accessible for 

 bacteria if soHd surfaces are present, (3) matter which apparently cannot be de- 

 composed by bacteria at all. In oligotrophic oceanic water, practically onl> the 

 two last-mentioned components are found. 

 If the hypothesis about the importance of solid surfaces for the bacteria to attack 

 the normal stock of " old " organic matter is correct, a dense bacterial flora should 

 always be found on the surface of plankton algae. As such a flora normally is not 

 found, the algae must be able to prevent the development. 



Pratt et al (1944) showed that the plankton alga Chlorclla produces antibiotics 

 with a high effect on bacterial growth. The normal absence of bacteria in any amount 

 from the surfaces of healthy plankton algae of all kinds indicates that all these spec.es 

 produce antibiotics. The ability to avoid bacterial settlement to any higher degree 

 by producing antibiotics may be considered to be a rather important condition, the 

 lack of which would make these species fairly unsuitable lor aquatic lite. 



